36.4. 一個完整的例子

這里是一個用 C 寫的非常簡單的觸發(fā)器例子。用過程語言寫的觸發(fā)器例子可以在文檔的過程語言那部分找到。

如果命令嘗試向trigf中插入空值，那么trigf會報告ttest表中的函數(shù)，并跳過實際的操作。因此觸發(fā)實際上執(zhí)行了一個非空約束，但不會中止事務(wù)。

First, the table definition: 首先，表定義：

CREATE TABLE ttest (
    x integer
);

這里是觸發(fā)器函數(shù)的源代碼：

#include "postgres.h"
#include "executor/spi.h"       /* 用 SPI 的時候要調(diào)用的頭文件*/
#include "commands/trigger.h"   /* ...和觸發(fā)器 */

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

extern Datum trigf(PG_FUNCTION_ARGS);

PG_FUNCTION_INFO_V1(trigf);

Datum
trigf(PG_FUNCTION_ARGS)
{
    TriggerData *trigdata = (TriggerData *) fcinfo->context;
    TupleDesc   tupdesc;
    HeapTuple   rettuple;
    char       *when;
    bool        checknull = false;
    bool        isnull;
    int         ret, i;

    /* 確信自己是作為觸發(fā)器調(diào)用的 */
    if (!CALLED_AS_TRIGGER(fcinfo))
        elog(ERROR, "trigf: not called by trigger manager");

    /*返回給執(zhí)行者的行 */
    if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
        rettuple = trigdata->tg_newtuple;
    else
        rettuple = trigdata->tg_trigtuple;

    /* 檢查 NULL 值 */
    if (!TRIGGER_FIRED_BY_DELETE(trigdata->tg_event)
        && TRIGGER_FIRED_BEFORE(trigdata->tg_event))
        checknull = true;

    if (TRIGGER_FIRED_BEFORE(trigdata->tg_event))
        when = "before";
    else
        when = "after ";

    tupdesc = trigdata->tg_relation->rd_att;

    /* 與 SPI 管理器連接 */
    if ((ret = SPI_connect()) < 0)
        elog(ERROR, "trigf (fired %s): SPI_connect returned %d", when, ret);

    /* 獲取表中的行數(shù)量 */
    ret = SPI_exec("SELECT count(*) FROM ttest", 0);

    if (ret < 0)
        elog(ERROR, "trigf (fired %s): SPI_exec returned %d", when, ret);

    /* count(*) 返回 int8 ，所以要小心轉(zhuǎn)換*/
    i = DatumGetInt64(SPI_getbinval(SPI_tuptable->vals[0],
                                    SPI_tuptable->tupdesc,
                                    1,
                                    &isnull));

    elog (INFO, "trigf (fired %s): there are %d rows in ttest", when, i);

    SPI_finish();

    if (checknull)
    {
        SPI_getbinval(rettuple, tupdesc, 1, &isnull);
        if (isnull)
            rettuple = NULL;
    }

    return PointerGetDatum(rettuple);
}

編譯完源代碼后(參閱Section 35.9.6)，聲明函數(shù)并創(chuàng)建觸發(fā)器：

CREATE FUNCTION trigf() RETURNS trigger
    AS 'filename'
    LANGUAGE C;

CREATE TRIGGER tbefore BEFORE INSERT OR UPDATE OR DELETE ON ttest
    FOR EACH ROW EXECUTE PROCEDURE trigf();

CREATE TRIGGER tafter AFTER INSERT OR UPDATE OR DELETE ON ttest
    FOR EACH ROW EXECUTE PROCEDURE trigf();

現(xiàn)在你可以測試觸發(fā)器的操作：

=> INSERT INTO ttest VALUES (NULL);
INFO:  trigf (fired before): there are 0 rows in ttest
INSERT 0 0

-- 插入被忽略，AFTER 觸發(fā)器沒有觸發(fā)

=> SELECT * FROM ttest;
 x
---
(0 rows)

=> INSERT INTO ttest VALUES (1);
INFO:  trigf (fired before): there are 0 rows in ttest
INFO:  trigf (fired after ): there are 1 rows in ttest
                                       ^^^^^^^^
                             還記得講過的關(guān)于可視性的原則嗎？
INSERT 167793 1
vac=> SELECT * FROM ttest;
 x
---
 1
(1 row)

=> INSERT INTO ttest SELECT x * 2 FROM ttest;
INFO:  trigf (fired before): there are 1 rows in ttest
INFO:  trigf (fired after ): there are 2 rows in ttest
                                       ^^^^^^
                             還記得講過的關(guān)于可視性的原則嗎？

INSERT 167794 1
=> SELECT * FROM ttest;
 x
---
 1
 2
(2 rows)

=> UPDATE ttest SET x = NULL WHERE x = 2;
INFO:  trigf (fired before): there are 2 rows in ttest
UPDATE 0
=> UPDATE ttest SET x = 4 WHERE x = 2;
INFO:  trigf (fired before): there are 2 rows in ttest
INFO:  trigf (fired after ): there are 2 rows in ttest
UPDATE 1
vac=> SELECT * FROM ttest;
 x
---
 1
 4
(2 rows)

=> DELETE FROM ttest;
INFO:  trigf (fired before): there are 2 rows in ttest
INFO:  trigf (fired before): there are 1 rows in ttest
INFO:  trigf (fired after ): there are 0 rows in ttest
INFO:  trigf (fired after ): there are 0 rows in ttest
                                       ^^^^^^
                             還記得講過的關(guān)于可視性的原則嗎？
DELETE 2
=> SELECT * FROM ttest;
 x
---
(0 rows)

在src/test/regress/regress.c和contrib/spi里還有更復(fù)雜的例子